In papermaking, a paper or paperboard product is sized by either adding a sizing agent to the pulp before forming the paper web (internal sizing) or by applying a sizing agent to the surface of the paper web (surface sizing). Sizing agents include starch solutions which may be applied to the paper at the wet end, at the size press or at the calender stack. Starch (C.sub.6 H.sub.10 O.sub.5) is a mixture of linear (amylose) and branched (amylopectin) polymers of .alpha.-D-glucopyranosyl units and the starch solutions may be characterized as cationic or anionic depending on the functional groups which may be attached to starch.
It is common practice to include pigments, plasticizers and hydrophobic emulsions with the starch solutions as surface sizing agents. A common hydrophobic emulsion used in papermaking is rosin (which may be selected from one or more of three main types: gum rosin, wood rosin and tall oil rosin) in its crude or refined state. The rosin may be hydrogenated, polymerized, modified with formaldehyde, etc., or fortified by addition of acid groups to increase the reactivity of the rosin. Typical examples of fortifying compounds which may be used to modify the rosin include fumaric acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride, citraconic acid, citraconic anhydride, acrylic acid, methacrylic acid and the like. Non-fortified rosin may also be used in combination with fortified rosin. Rosin-based sizing agents may have either an anionic or cationic charge characteristic for use as both an internal size and a surface size.
Commercially available rosin is a glassy, amorphous material which softens at temperatures between about 75.degree. and 95.degree. C. The primary components of rosin are various monocarboxylic resin acids of alkylated hydrophenanthrene structures having the empirical formula C.sub.19 H.sub.(27-33) COOH. Resin acids may be divided into two general structural types, abietic and pimaric acids. Because the bulky hydrocarbon portion of a resin acid provides considerable shielding of the relatively small polar carboxyl group of the molecule, rosins can serve as a very effective hydrophobic coating for a cellulosic surface.
The rosin emulsion may be prepared by reacting rosin with an aqueous alkali. The degree of neutralization of the resin acids of the rosin may vary from 5% to 100% depending on the physical state desired for the commercial emulsion product.
An aqueous emulsion of rosin solids consists of suspended particles having a diameter of 0.1 to 0.5.mu., soap micelles having a diameter of about 50 .ANG. and individual resinate ions. The pH of the emulsified rosin is typically in the range of from about 3 to about 5.
Conventionally, the rosin emulsion used as a component of a surface sizing agent is an anionic rosin emulsion which may be added to an anionic size-press starch solution or a cationic rosin emulsion which may be added to a cationic starch solution. However, with either of these systems, poor sizing efficiency and/or unsatisfactory runnability have been observed. Deposit formation on the size-press, drier cans or calender rolls is one of the principal runnability problems reported with such sizing agents.
Accordingly, it is an object of the invention to provide an improved rosin emulsion.
Another object of the invention is to improve the surface characteristics of paper coated with a rosin-based sizing agent.
Yet another object of the invention is to increase the toner adhesion characteristics of a rosin-based surface-sized base-stock.
A further object of the invention is to provide a rosin-based sizing agent which exhibits good sizing efficiency and satisfactory runnability and which is useable under alkaline or acidic papermaking conditions.